1. Field of the Invention
The present invention relates to a tubular element for use in a heat-exchanger for example for use in a radiator, car-heater, intercooler or the like in an automobile.
2. Description of the Prior Art
Aluminum heat exchangers are known for the above-mentioned uses which comprise tubular elements which allow a heat-exchanging medium to flow therethrough. These heat-exchangers require a high-corrosion resistance and good mechanical strength in order to provide an adequate lifetime which is typically considered to be around 10 years. It is known to employ a controlled atmosphere braze (CAB) furnace process to manufacture such heat-exchangers from the tubular elements of the alloy "AA3003" which is relatively corrosion resistant and the composition of which is specified by the Aluminum Association. The "AA3003" alloy is used for the tube, header and sidewall of the exchangers and modifications to the "AA3003" alloy composition have been made in order to achieve both corrosion resistance and strength. However, the brazing process carried out on "AA3003" tubular elements can cause secondary effects such as silicon diffusion and fin erosion which has limited the performance of such tubular elements in the pressure and thermal cycles and in corrosive tests for production validation.
In addition, the heat-exchange medium which is employed in heat-exchangers is generally water which may include impurities mixed with engine coolants and atmospheric contaminants. These provoke corrosion susceptibility of the heat-exchanger tubular elements during normal use. The tubular elements of known heat-exchangers comprise an inner clad layer which has a sacrificial character, that is a nobler corrosion potential than the core of the tubular element and an outer clad layer, which is a brazing layer, for securing fin members to the tubular elements. The inner clad layer is intended to protect the heat-exchanger tube and other components against corrosion. The core material is also required to exhibit good resistance to silicon penetration by diffusion, the diffusion being dependent upon the brazing time, the brazing temperature and the silicon content in the brazing layer.
U.S. Pat. No. 4,991,647 discloses a heat-exchanger comprising tubular elements made of a first aluminum alloy and fin members of a second aluminum alloy. The first aluminum alloy comprises 0.05 to 1.0 wt % of Mg, 0.2 to 1.2 wt % of Si, 0.2 to 1.5 wt % of Mn, 0.01 to 0.5 wt % of Fe and the balance aluminum. The second aluminum alloy comprises 0.05 to 1.0 wt % of Mg, 0.2 to 1.2 wt % of Si, 0.2 to 1.5 wt % of Mn 0.01 to 0.5 wt % of Fe, at least one of 0.01 to 1.0 wt % of In and 0.1 to 2.0 wt % Zn and the balance aluminum.